1.4: Macromolecules Flashcards
What is a cell?
A cell is the smallest, most basic biological unit of life.
What are macromolecules?
—complex molecules that are composed of smaller subunits.
There are four main types of macromolecules found in cells:
(1) proteins
(2) nucleic acids
(3) lipids
(4) carbohydrates.
Proteins
Proteins are polymers (a large molecule comprised of many smaller subunits) of amino acids, and they are the most abundant form of macromolecule. Proteins have a variety of functions in the life of a cell. Proteins may facilitate the movement of materials in or out of a cell. Some can act as enzymes that catalyze, or speed up, biochemical processes. Others play a structural role, while other proteins, such as filaments, enable movement. There is remarkable diversity within cells because of the way proteins are formed by varying combinations of amino acids. Note: There are 20 different amino acids, and the order and length in which they are assembled give rise to different protein sizes, structures, and functions. Each protein has its own unique sequence of amino acids, which is known as the primary structure of the protein. The largest known protein, titin, is made up of about 33,000 amino acids! Of the twenty amino acids, it should also be noted that nine are considered essential amino acids, as the human body cannot produce them—they must instead be taken in from the environment through other sources (i.e.) the food you eat.
Nucleic Acids
Nucleic acids are chemical molecules that carry genetic information within the cell. There are two major types of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). DNA contains a vast amount of hereditary information and is responsible for the inheritable characteristics of living organisms. RNA is responsible for deciphering the hereditary information in DNA and using it to synthesize proteins. While both DNA and RNA can be found in the nucleus (center of the cell), only RNA can leave the nucleus. DNA molecules are coiled and twisted into a highly compact form to fit in the nucleus of the cell. Deoxyribonucleic acid (DNA) is made from building-block molecules called nucleotides. Each nucleotide has three parts, a nitrogenous (nitrogen-containing) base, a sugar called deoxyribose, and a phosphate group. There are four kinds of nitrogenous bases that can occur in DNA nucleotides: adenine or guanine (purine bases) and cytosine or thymine (pyrimidine bases). The bases can be abbreviated, A, G, C, and T, respectively. Structurally, the sugar and phosphate portions form the backbone of a DNA strand, while the nitrogenous base protrudes outward from the deoxyribose sugar. Strands of DNA form a double helix, meaning there are two strands held together by hydrogen bonding. As shown in Figure 1.1 below, the top strand complements and pairs with the corresponding bottom strand. Hydrogen bonds are formed between adenine (A) on one strand and the thymine (T) on the complementary strand, while guanine (G) on one strand pairs with cytosine (C) on the other strand, and vice versa. For this reason [adenine and thymine] or [guanine and cytosine] are called complementary base pairs. Further, base pairings between A-T results in two hydrogen bonds being formed, while G-C pairings results in the formation of three hydrogen bonds.
Ribonucleic acid (RNA)
is also composed of nucleotides and shares many similar properties to DNA, with some key distinctions. As the name suggests, RNA contains the sugar ribose rather than deoxyribose. RNA is always single-stranded, whereas DNA is double-stranded. While RNA contains the bases adenine, guanine, and cytosine, it does not utilize the pyrimidine base thymine. Instead, RNA contains uracil (U), which still pairs with adenine (A). Figure 1.2 below illustrates the difference in base pairings within an RNA stand.
Lipids
Lipids are mainly composed of hydrophobic (water fearing) hydrocarbons (hydrogen + carbon atoms). The primary role of lipids in cells is forming the foundation of the plasma membrane. In its most basic form, the plasma membrane serves the cell as a surrounding barrier that separates the inside of the cell from the outside surrounding environment. The plasma membrane also restricts the movement of materials (water, nutrients, etc.) either in or out of the cell, thus allowing a cell to absorb and keep what is needed (influx) while also preventing the escape (out flux) of essential nutrients. In most organisms, the membrane consists of a lipid bilayer, meaning there are two layers of lipids stacked on top of each other, with the hydrophobic tail regions pointing inward, as shown in Figure 1.6. However, the plasma membrane is not impervious. Additional macromolecules, such as polysaccharides and proteins, are often associated with the membrane and aid in controlling the movement of materials in and out of the cell.
Carbohydrates
Carbohydrate literally means “watered carbon”, as it is a molecule made of carbon (C), hydrogen (H), and oxygen (O) atoms. Its chemical formula is (CH 2 O) n where the n denotes that the number of carbon and oxygen numbers is always equal with the number of hydrogen atoms double. For instance, the sugar glucose consists of six carbons, twelve hydrogens, and six oxygens atoms, and is abbreviated as C 6 H 12 O 6 . Carbohydrates are classified into 3 groups based on the number monomers of sugar, or saccharide, present in the molecule:
Monosaccharide – The prefix “mono” means one, so a monosaccharide contains 1 single sugar unit. Glucose, fructose and galactose are examples. Monosaccharides can exist as either linear or ring structures, with cells favoring the ring like structure for molecules like DNA, RNA, and storage molecules.
Disaccharide – The prefix “di” means two, so a disaccharide contains 2 monosaccharides are joined together. Sucrose (C 12 H 22 O 11 ) is an example of a disaccharide formed by joining glucose and fructose sugar molecules together. Other disaccharides are maltose and lactose. Disaccharide molecules are broken into monosaccharides by hydrolysis reactions.
Polysaccharide – The prefix “poly” means many, so a polysaccharide is 3 or more monosaccharides linked together. Polysaccharides made of glucose molecules are important for cellular energy storage. In plants, these polysaccharides are called starch, and in animals it is referred to as glycogen. Polysaccharides also play a role in cellular structure. The polysaccharide cellulose forms the structural cell walls in plants, whereas the sugar chitin is the primary structural component of fungal (yeast, mold and mushrooms) cell walls. Bacterial cell walls are made of a polysaccharide modified by amino acids, which results in the peptidoglycan molecule.
